The present invention relates to improved liquid crystal electro-optical devices, and more particularly relates to ferroelectric liquid crystal devices employing a new and improved driving mode.
Heretofore, it has been known to utilize twisted nematic liquid crystals in the derivation of electro-optical displays. The liquid crystal materials are employed in layer form and a number of pixels are defined in the liquid crystal layers by provision of a matrix electrode arrangement contiguous to the liquid crystal layer. However, due to the occurence of crosstalk between adjacent pixels during operation in a time multiplexed mode, the number of pixels that can be achieved per unit display area, i.e. the pixel density, is substantially limited.
Switching in such a display may be performed by means of thin film transistors provided for each other, such a driving arrangement being called an active matrix system. However, because of the complexities of the manufacturing process, it is very difficult to produce such a display with a large area and at the same time to obtain cost reduction.
In an attempt to solve the above shortcomings of prior art devices, Clark et al. have proposed a ferroelectric liquid crystal device in their U.S. Pat. No. 4,367,924 and FIG. 1 of the accompanying drawings is an explanatory schematic diagram showing the action of liquid crystal molecules in such a prior art device. A ferroelectric liquid crystal is interposed between a pair of glass substrates 11 and 11' each provided with an electrode arrangement made of In.sub.2 O.sub.3, SnO.sub.2 or ITO (Indium Tin Oxide) on the inner surface thereof. The liquid crystal material is arranged between the substrates so that each molecular layer 12 is normal to the substrates as illustrated in the figure. The operating phase of the liquid crystal material is chiral smectic C and desirably the material has this phase at room temperature. The liquid crystal molecules can adopt two stable positions I and II which inclined at angles .theta. and -.theta. to the layer normal as shown in FIG. 2.
The position of the molecules can be switched between these two stable positions in dependence upon an externally applied electric field directed normally to the substrates, and in this manner visual images can be constructed based on differential birefringence between pixels in different states. One feature of this type of display device is its bistablity which arises by virtue of that fact that the position of each liquid crystal molecule will be maintained even after the applied electric field has been removed and will remain so until another electric field is applied anew in the opposite sense. In other words, such display devices can function as non-volatile memory elements.
While this proposal of Clark et al. seems promising as regards the derivation of a new type of liquid crystal device capable of displaying non-volatile information at a high pixel density, there is a critical disadvantage from the viewpoint of production. The pair of glass substrates between which liquid crystal material is disposed have to be spaced apart from each other at such a small distance as to ensure unwinding of the helical liquid crystal molecules. The distance between the substrates thus has to be of the order of the pitch of the helix, e.g. few micrometer or thereabouts. The performance of liquid crystal devices of this type is thus very sensitive to the uniformity of the distance between substrates. For this reason, it is anticipated that it will be very difficult to ensure the uniformity of such a narrow gap in mass-production of such devices.